Method of making glass ceramic and product



United States Patent Ofiice 3,352,698 Patented Nov. 14, 1967 3,352,698 METHOD OF MAKING GLASS CERAMIC AND PRODUCT Peter William McMillan, Staiford, and Donald Christopher Lawton, Seabridge, Newcastle, England, assiguors to The English Electric Company Limited, London,

England, a British company No Drawing. Filed Jan. 9, 1967, Ser. No. 607,881 Claims priority, application Great Britain, Nov. 30, 1962,

45,318/62; May 29, 1963, 21,558/63 6 Claims. (Cl. 10639) ABSTRACT OF THE DISCLOSURE A strong, opaque glass-ceramic having a low thermal expansion coefiicient is produced by heat-treatment from a glass of the following composition, in percentages by weight: Si 59.0-62.0; Li O 4.4-5.7; K 0 3.3-4.2; ZnO 5.1-5.3; and A1 0 17.8-18.6; by the addition as nucleating agents of 1.5-7.5 percent ZrO and 1.5-6.0 percent M00 In the absence of M00 the amount of ZrO which can be incorporated in this glass composition, without the occurrence of undissolved particles of ZrO which create weaknesses in the product, is insutficient to act as a satisfactory nucleating agent. The addition of M00 improves the solubility of the ZrO in the glass composition, and makes possible satisfactory nucleation resulting in strong, low-expansion glass-ceramics.

' This application is a continuation-in-part of US. Ser. No. 550,573, filed May 16, 1966, now abandoned, which in turn was a continuation-in-part of U5. Ser. No. 298,- 484, filed July 29, 1963 (now abandoned). Priority is claimed of applications in Great Britain No. 45,318/ 62, filed Nov. 30, 1962 and No. 21,558/63, filed May 29, 1963.

This invention relates to glass-ceramics.

It is known from US. Patent No. 3,238,085 (Hayami et al.) to make glass-ceramic materials of the glass system using As O in the range 2-8 percent as the essential nucleating agent. This specification also states that the addition of a small quantity (2-8 percent) of Moo -0r W0 to the glass composition has the eifect of promoting crystallization. However none of the six examples includes M00 and the eifect of adding this substance cannot therefore be'definitely ascertained. It is noteworthy, however, that each of Examples 3, 4 and 5 of Hayami et al., which contain both AS303 and W0 have substantially lower bending strengths than Examples 1 and 2 which contain A5 0 but not W0 or M00 Since Hayami et al. treats W0 and M00 as equivalent, it may .be inferred that the use of a combination of AS203 and M00 as the nucleating agent will result in a lower strength than the use of As O alone.

This inference is supported by the disclosure of US. Patent 3,117,881 (Henry et al.). This specification deals with a glass system which differs from that of the present application in containing MgO as an essential constituent, whereas applicants glass system specifically excludes MgO.

Nevertheless, comparison of Examples 31 and 32 of Henry et a1. shows that these examples are identical in composition except that Example 31 has 7.0 percent ZrO whereas Example 32 has 8.0 percent, and Example 31 has 1.8 percent TiO whereas Example 32 has 0.8 percent M00 the heat-treatments are also very similar. Yet, whereas Example 31 has a flexural strength of 160,730

p.s.i., Example 32 has a flexural strength of only 32,340 p.s.1.

It must be concluded that, with this glass composition, the addition of M00 has an adverse effect, especially since it results in one of the lowest values of flexural strength for any of the thirty-nine examples quoted by Henry et al.

US. Patent No. 3,241,985 (Kuwayama) discloses the preparation of glass-ceramics of the SiO -Li O-Al O glass system using ZrO as the nucleating agent, but in no case using M00 It will also be noted that although applicants glass system is the same as that of Kuwayama, the basic glass is separate and distinct in that applicants percentage range does not overlap that of Kuwayama either in respect of SiO or K 0, or ZnO.

The glass composition of the product of the present invention is as follows, in percentages by weight:

SiO 59.0-62.0; Li O 4.4-5.7; K 0 3.3-4.2; ZnO 5.1- 5.3; A1 0 17.8-18.6; M00 1.5-6.0; and Zr0 1.5-7.5.

It is an object of the present invention to produce a high-strength glass-ceramic having a low coefiicient of thermal expansion.

It is well known in the glass-ceramic art that while it is possible to produce high-strength glass-ceramics (e.g. those of US. Patent 3,117,881 Henry et al.), and also to produce glass-ceramics having low coefiicients of thermal expansion (e.g. those of US. Patent 3,241,985 Kuwayama), it is more dificult to produce glass-ceramics having both high strengths and low coetficients of thermal expansion. Such glass-ceramics have been found to be highly resistant to thermal shock.

It is therefore another object of the present invention to produce a glass-ceramic of novel composition which is highly resistant to thermal shock.

For certain applications it is advantageous to produce glass-ceramics which are opaque, rather than translucent or transparent, for example in the production of containers for food or drugs which are adversely affected by exposure to light. Such containers must be strong, to withstand handling, and it is an advantage if they are resistant to thermal shock.

It is therefore a further object of the present invention to provide an opaque glass-ceramic which is strong and has a low thermal expansion coeflicient, whereby it is resistant to thermal shock.

Applicants have found that with glasses containing in weight percentages SiO 59.0-62.0; Li O 4.4-5.7; A1 0 17.8-18.6;ZnO 5.1-5.3; and K 0 3.3-4.2, a nucleating agent is necessary for the production of satisfactory glassceramics. As a result of the limited solubility of ZrO in glass of this composition, the amount of ZrO which can be incorporated, when used as the sole nucleating agent, is insuflicient to act satisfactorily, and glass-ceramics of this composition to which ZrO has been added for the purpose of acting as a nucleating agent have been found to contain undissolved ZrO which results in a very weak product.

However, applicants have found that, surprisingly, the use of M00, increases the solubility of Zr0 in the glass composition, and therefore enables sufficient ZrO to be used to act as a nucleating agent, while at the same time avoiding the presence of undissolved particles and the coarse structure hitherto found.

It will .be apparent that neither ZrO nor M00 when used alone will enable comparable strong low-expansion glass-ceramics to be produced.

Typical processes for the formation of glass-ceramics way of example.

Suitable raw materials are melted to form a glass of the desired composition. The raw materials used to introduce the stated oxides may include:

Raw material: Oxide Ground quartz or glass-making sand SiO Lithium carbonate Li O Potassium carbonate K Zinc oxide ZnO Aluminium oxide or hydrated alumina A1 0 Molybdenum trioxide M00 Zirconium silicate ZrO SiO Sodium carbonate N320 Calcium carbonate CaO Boric acid B203 The batch materials are thoroughly mixed before melting and are melted in crucibles of recrystallized alumina or other suitable refractory material at a temperature of 1500 to 1600" C. in a neutral or slightly reducing atmosphere. This may be achieved, where the furnace atmosphere is strongly reducing, by fitting. covers on the crucibles.

The composition of the glass consists essentially of the following ranges of constituents, in percentages by Weight: SiO 59.0-62.0; Li O 4.4-5.7; K 0 3.3-4.2; ZnO' 5.1-5.3; A1 0 17.8-18.6; M00 1.5-6.0; and Zr0 1.5-7.5.

As is disclosed and stated in the parent application Ser. No. 298,484, it is seen that the preferred range of crystallization regulators present in the glass is in the proportion of 1.5 to 6.0 percent of molybdenum trioxide and preferably from 2.0 to 6.0 percent of this substance, and another preferred range of percentage composition by weight of molybdenum trioxide (M00 1.5 to 5.0.

The K 0 may be replaced by Na or alternatively both K 0 and Na O may be present within the range specified; CaO and/or B 0 may be present in the range 0-5 percent.

It is preferred that the glasses contain approximately equal amounts of the crystallization regulators M00 and Z and the preferred total content of the two is approximately 10 percent.

The presence of impurities introduced in the batch materials is permissible provided that they do notexceed the limits normally accepted in glass-making processes.

The glasses are then refined and worked by the normal glass-working methods such as casting, drawing or pressing. The articles so produced may be annealed at a temperature in the range 550 to 650 C. if it is desired to store them before heattreatrnent, or alternatively the articles may be heat-treated immediately.

The heat-treatment to devitrify the glasses in a controlled manner comprises first heating the articles at a rate not exceeding 10 C. per minute, and preferably at 4 to 5 C. per minute, to a temperature which lies between a temperature C. below the softening point (Mg point) of the glass and a temperature50 C. above the softening point. In this context the Mg point corresponds to the dilatometric softening temperature at which the glass has a viscosity of 10 to 10 poises (see, for example, F. V. Tooley Handbook of Glass Manufacture, pp. 40-41). This temperature is held for at least thirty minutes.

The articles are. then further heated at the same rate to a final crystallization temperature in the range 900 to 1100" C. and this temperature is maintained for at least one hour. The exact temperature depends largely on the total alkali content of the glass. The articles are then allowed to cool at not more than 10 C. per minute and preferably at the normal cooling rate of the furnace.

The heat-treatment may be modified by including an intermediate holding stage at a temperature in the range between the softening point and 980 C. and a further holding stage between 900 C. and the final crystallization temperature. The articles may be maintained at these holding temperatures for any desired length of time and it is found that the strength and coefficient of linear thermal expansion of the resulting ceramic products may be varied to a certain extent by modifying the heat-treat ment in this way.

A number of examples of the glass composition, heat-, treatment conditions, and some of the resulting physical properties of glass-ceramics in accordance with the invention are given in the following tables:

TABLE I Composition, percentage by weight Example A B C D E Heat-treatment Example A1 A2 B C D E Temp., e o 580 580 580 Time, ms. 3 3 3 Temp., C 980 980 980 Time, hrs 1 1 1 Properties Modulus of rupture,

13.5.1 19,000 28, 000 14, 200 19, 900 Thermal expansion coefiieient X10 (20- 500 C.) 18.7 20 19.4 14. 5

1 See text.

TABLE II Composition, percentage by weight F o H I J K Heat-treatment Temp, C 580 580 900 900 580 Time, hrs 3 3 1 1 3 Temp., 0 980 980 1,100 1,100 980 Time, hrs 1 1 1 1 1 Properties Modulus of rupture,

p.s.i 18,100 22,400 0, 950 4,750 19,500 Thermal expansion coellicient 21. 0 24. 2 l6. 3 l2. 6 21. 4

It is believed that the heat-treatment at the lower tem- Heat-treatment Schedule I perature leads to the formation of sub-microscopic crys- 5 a o tals of complex compounds containing both zirconium o 3 Temperature ralsed 5 P mmute to 580 and molybdenum dispersed throughout the glassy matrix, Temperature of 580 mamtalfged for and this will be referred to as the nucleation stage. g gf further ralsed at 5 P mlnute The heat-treatment at the higher temperature is known d T Q o as the crystallization stage, and causes the conversion of emperamre of 980 mamtaolned for one'hollra substantial part of the composition from a glass to a (e) Temperature reduced at 540 P mlnute t0 crystalline structure. The proportion which is thus conroom temperatureverted may vary from a value of under 50 percent to a Heamreatmem Schedule H value of nearly 100 percent, and the physical properties may be varied to some extent by varying the proportion 40 (21) Temperature raised at 45 C. per minute to 600 C. converted by a suitable choice of heat-treatment times and Temperature of 600 C. maintained for thirty temperatures. The properties which can be obtaind are, minutes. of course, dependent on the glass composition. T mp r re further raised at 4-5 C. per minute All the examples of glass-ceramics given above were to 800 C. opaque and good electrical insulators and, as can be seen 1) Temperature of 800 C. maintained for one hour. from the tables, Examples I and J have moderately high (6) Temperature further raised to 1,000 C. cross-breaking strengths and the remainder have high Temperature of 1,000" n ained for one hour. strengths by the standards of homogeneous glass-ceramics, (8) Temperature further raised to taking into account the method of testing. In the case of (P) Temperature of 1,050 tain d f r One hour. Examples I and J the strength is not less than that of T p ature r du ed at5-10" C. per minute to room the glass before devitrification and in all other cases the strength is substantially greater than, i.e. about three or four times, that of the undevitrified glass. Coefiicients of thermal expansion of up to 25x10" may be considered as low-expansion materials in the art, and it will be seen that the measured values for the examples are all below this figure.

Specific examples using Compositions A and B of Table I above will now be described in greater detail, for a fuller understanding of the invention. A mixture of raw materials to give a glass of the following percentage composition by weight (Composition A) was melted at 1560 C. in a recrystallized alumina crucible:

After refining, the glass was shaped by normal glassworking processes and the articles so formed were then annealed at 600 C. Samples of the glass of Composition A were heat-treated according to two heat-treatment schedules as follows:

temperature.

The resulting glass ceramics were found to be mechanically strong, the articles produced by heat-treatment Schedule I having a modulus of rupture of 19,000 lbs. per square inch and a coefiicient of linear thermal expansion of 18.7 10- per degree C. Those produced by heat-treatment Schedule II had a modulus of rupture of 21,600 lbs. per square inch and a coeflicient of linear thermal expansion of 235x10 per degree C.

Another mixture of raw materials to give a glass of the following composition in percentages by weight (Composition B) was melted in a recrystallized alumina crucible at.1,560 C.

The glass of Composition B, after refining, was shaped by normal glass-working processes and was annealed at 7 600 C. Samples of the glass were heat-treated according to the following. schedule:

(a) Temperature raised at C. per minute to 580 C.

(b) Temperature of 580 C. maintained for three hours.

(c) Temperature further raised at 5 C. per minute to (d) Temperature maintained at 980 C. for one hour.

(e) Temperature reduced at 5-10 C. per minute to room temperature.

The resulting articles were mechanically strong, having a modulus of rupture of 28,000 lbs. per square inch and a coefiicient of linear thermal expansion of 20 10 per degree C.

What we claim as our invention and desire to secure by Letters Patent is:

1. In a method of making a glass-ceramic product including the steps of melting glass batch materials to form a glass, said materials including Zr-O as a nucleating agent, and heat-treating the resulting glass by heating it to a crystallization temperature in the range 9001,l00 C. until a product which is at least partially microcrystalline is obtained, the improvement which comprises selecting batch materials to form a glass consisting essentially of,

and adding materials to the batch to form in the glass- ZrO in the range 1.5-7.5 weight percent and M00 in the range 1.5-5.0 weight percent, whereby the presence of M00 enables Zr0 in the range given to be completely dissolved in the glass and to act as a nucleating agent, and whereby the resulting glass-ceramic produced is opaque, has a coeflicient of thermal expansion not exceeding 25 X 10 (20-500 C.) and has a cross-breaking strength not less than that of the glass before devitrification.

2. A method as claimed in claim 1, wherein the glass is heat-treated at a nucleation temperature of substantially 580 C. and then at a crystallization temperature of substantially 980 0., whereby the resulting glass-ceramic. has a cross-breaking strength which is substantially greater than that of the glass before devitrification.

3. A method as claimed in claim 1, wherein said batch materials, apart from ZrO and M00 consist essentially of, in percentages by weight:

4. A product by the process of claim 1. 5. A product by the process of claim 2. 6. A product by the process of claim 3.

References Cited FOREIGN PATENTS 1,099,135 2/1961 Germany.

DONALL H. SYLVESTER, Primary Examiner. G. R. MYERS, Assistant Examiner. 

1. IN A METHOD OF MAKING A GLASS-CERAMIC PRODUCT INCLUDING THE STEPS OF MELTING GLASS BATCH MATERIALS TO FORM A GLASS, SAID MATERIALS INCLUDING ZRO2 AS A NUCLEATING AGENT, AND HEAT-TREATING THE RESULTING GLASS BY HEATING IT TO A CRYSTALLIZATION TEMPERATURE IN THE RANGE 900-1,100*C. UNTIL A PRODUCT WHICH IS AT LEAST PARTIALLY MICROCRYSTALLINE IS OBTAINED, THE IMPROVEMENT WHICH COMPRISES SELECTING BATCH MATERIALS TO FORM A GLASS CONSISTING ESSENTIALLY OF, IN PERCENTAGES BY WEIGHT: 